32 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



Frequently their former presence is attested only by the greater abundance in 

 certain areas of ferritic minerals, which form a rude pseudomorph after the 

 original crystal. Sometimes only the outline of the original is preserved, and 

 rarely the original lines of cleavage can be traced. Often, on the other hand, 

 the outline has been lost, and the decomposition products are bunched together 

 so rudely that the primary mineral can only be guessed at. 



The feldspar also is sometimes so completely altered to a felt of secondary 

 minerals, entirely similar to those resulting from the decomposition of the ground- 

 mass, that its former existence can not be determined without careful observation. 

 If viewed by reflected light the outlines of the feldspar crystals can sometimes 

 be seen. Frequently the secondary minerals within the area of the original feld- 

 spar are of slightly coarser grain than those without. The feldspar has altered 

 essentially to quartz and sericite. Alteration of the feldspar to adularia or valen- 

 cianite (a variety of orthoclase) is also widespread and important. The decompo- 

 sition products not infrequently include kaolin, and occasionally calcite, chlorite, 

 and epidote. 



The groundmass undergoes the same decomposition processes as the porphy- 

 ritic crystals, becoming generally a felt} 7 aggregate which is composed of secondary 

 quartz and sericite, but which includes some pyrite, siderite, and limonite, and 

 sometimes a little kaolin. By a rarer process of alteration chlorite and calcite 

 are formed. 



As a result of these alteration processes the rock is usually more or less 

 completely altered to an aggregate which is composed of quartz and sericite, and 

 which usually includes some pyrite and siderite, and frequently adularia, kaolin, 

 and the iron oxides. Chlorite and calcite are not so common, but one or both of 

 these minerals may be very abundant. They indicate a process of decomposition 

 different from the ordinary. Chlorite may occur in a rock without calcite, and 

 vice versa. In one specimen studied, quartz and chlorite were the chief products 

 of decomposition; in another, quartz, sericite, and chlorite. As a rule, however, 

 the rocks may be divided according to their processes of decomposition, as 

 follows: 



1. Quartz-sericite-adularia-pyrite-siderite rocks; most abundant, and most closely connected 

 with the metalliferous veins. 



2. Quartz-sericite-kaolin-iron oxides rock; not infrequent; probably a modification of No. 1. 

 Usually plainly associated with some fault or other underground water channel. 



3. Chlorite-calcite rock; not associated with the ores. 



Location. The earlier andesite outcrops in only a limited area, being chiefly 

 confined to Mizpah Hill and Gold Hill. It has been proved to occur extensively, 

 however, underneath later lavas. 



